This invention is in the field of plant molecular biology. More specifically, this invention pertains to nucleic acid fragments encoding enzymes involved in biosynthesis and utilization of branched chain amino acids in plants and seeds.
Many vertebrates, including man, lack the ability to manufacture a number of amino acids and therefore require these amino acids preformed in their diet. These are called essential amino acids. Plants are able to synthesize all twenty amino acids and serve as the ultimate source of the essential amino acids for humans and animals. Thus, the ability to manipulate the production and accumulation of the essential amino acids in plants would be of considerable importance and value. Furthermore, the inability of animals to synthesize these amino acids provides a useful distinction between animal and plant cellular metabolism. This can be exploited for the discovery of herbicidal chemical compounds that target enzymes in the plant biosynthetic pathways of the essential amino acids and thus have low toxicity to animals.
The branched-chain amino acids leucine, isoleucine and valine are three of the essential amino acids. Biosynthesis of these amino acids proceeds, in part, via the common enzymes acetolactate synthase, acetohydroxyacid reductoisomerase, dihydroxyacid dehydratase and branched chain amino acid aminotransferase, and in part via enzymes specific for one of the amino acids, threonine dehydratase (isoleucine), and xcex1-isopropylmalate synthase, 3-isopropylmalate dehydratase and xcex2-isopropylmalate dehydrogenase (leucine). Regulation of the biosynthesis of each member of this family in plants is interconnected (see FIG. 1), but understanding of the control is poor.
Few of the genes encoding enzymes that regulate this pathway in plants, especially corn, soybeans, rice and wheat, have been isolated and sequenced. For example, no plant genes have yet been reported for dihydroxyacid dehydratase, branched chain amino acid aminotransferase or 3-isopropylmalate dehydratase. Accordingly, the availability of nucleic acid sequences encoding all or a portion of these enzymes would facilitate studies to better understand the cellular control of the pathway, provide genetic tools for the manipulation of the pathway and provide a means to evaluate chemical compounds for their ability to inhibit the activity of these plant enzymes.
The instant invention relates to isolated nucleic acid fragments encoding plant enzymes involved in biosynthesis and utilization of branched-chain amino acids. Specifically, this invention concerns an isolated nucleic acid fragment encoding a dihydroxyacid dehydratase, a branched chain amino acid aminotransferase, a leuC subunit of 3-isopropylmalate dehydratase, or a leuD subunit of 3-isopropylmalate dehydratase. In addition, this invention relates to a nucleic acid fragment that is complementary to the nucleic acid fragment encoding dihydroxyacid dehydratase, branched chain amino acid aminotransferase, leuC subunit of 3-isopropylmalate dehydratase, or leuD subunit of 3-isopropylmalate dehydratase.
An additional embodiment of the instant invention pertains to a polypeptide encoding all or a substantial portion of a plant branched-chain amino acid biosynthetic enzyme selected from the group consisting of dihydroxyacid dehydratase, branched chain amino acid aminotransferase, leuC subunit of 3-isopropylmalate dehydratase, and leuD subunit of 3-isopropylmalate dehydratase.
In another embodiment, the instant invention relates to a chimeric gene encoding a dihydroxyacid dehydratase, a branched chain amino acid aminotransferase, a leuC subunit of 3-isopropylmalate dehydratase, or a leuD subunit of 3-isopropylmalate dehydratase, or to a chimeric gene that comprises a nucleic acid fragment that is complementary to a nucleic acid fragment encoding a dihydroxyacid dehydratase, a branched chain amino acid amino-transferase, a leuC subunit of 3-isopropylmalate dehydratase, or a leuD subunit of 3-isopropylmalate dehydratase, operably linked to suitable regulatory sequences, wherein expression of the chimeric gene results in production of levels of the encoded protein in a transformed host cell that is altered (i.e., increased or decreased) from the level produced in an untransformed host cell.
In a further embodiment, the instant invention concerns a transformed host cell comprising in its genome a chimeric gene encoding a dihydroxyacid dehydratase, a branched chain amino acid aminotransferase, a leuC subunit of 3-isopropylmalate dehydratase, or a leuD subunit of 3-isopropylmalate dehydratase, operably linked to suitable regulatory sequences. Expression of the chimeric gene results in production of altered levels of the encoded protein in the transformed host cell. The transformed host cell can be of eukaryotic or prokaryotic origin, and include cells derived from higher plants and microorganisms. The invention also includes transformed plants that arise from transformed host cells of higher plants, and seeds derived from such transformed plants.
An additional embodiment of the instant invention concerns a method of altering the level of expression of a dihydroxyacid dehydratase, a branched chain amino acid aminotransferase, a leuC subunit of 3-isopropylmalate dehydratase, or a leuD subunit of 3-isopropylmalate dehydratase in a transformed host cell comprising: a) transforming a host cell with a chimeric gene comprising a nucleic acid fragment encoding a dihydroxyacid dehydratase, a branched chain amino acid aminotransferase, a leuC subunit of 3-isopropylmalate dehydratase, or a leuD subunit of 3-isopropylmalate dehydratase; and b) growing the transformed host cell under conditions that are suitable for expression of the chimeric gene wherein expression of the chimeric gene results in production of altered levels of dihydroxyacid dehydratase, branched chain amino acid aminotransferase, leuC subunit of 3-isopropylmalate dehydratase, or leuD subunit of 3-isopropylmalate dehydratase in the transformed host cell.
An addition embodiment of the instant invention concerns a method for obtaining a nucleic acid fragment encoding all or a substantial portion of an amino acid sequence encoding a dihydroxyacid dehydratase, a branched chain amino acid aminotransferase, a leuC subunit of 3-isopropylmalate dehydratase, or a leuD subunit of 3-isopropylmalate dehydratase.
A further embodiment of the instant invention is a method for evaluating at least one compound for its ability to inhibit the activity of a dihydroxyacid dehydratase, a branched chain amino acid aminotransferase, a leuC subunit of 3-isopropylmalate dehydratase, or a leuD subunit of 3-isopropylmalate dehydratase, the method comprising the steps of: (a) transforming a host cell with a chimeric gene comprising a nucleic acid fragment encoding a dihydroxyacid dehydratase, a branched chain amino acid aminotransferase, a leuC subunit of 3-isopropylmalate dehydratase, or a leuD subunit of 3-isopropylmalate dehydratase, operably linked to suitable regulatory sequences; (b) growing the transformed host cell under conditions that are suitable for expression of the chimeric gene wherein expression of the chimeric gene results in production of dihydroxyacid dehydratase, a branched chain amino acid aminotransferase, a leuC subunit of 3-isopropylmalate dehydratase, or a leuD subunit of 3-isopropylmalate dehydratase in the transformed host cell; (c) optionally purifying the dihydroxyacid dehydratase, the branched chain amino acid aminotransferase, the leuC subunit of 3-isopropylmalate dehydratase, or the leuD subunit of 3-isopropylmalate dehydratase expressed by the transformed host cell; (d) treating the dihydroxyacid dehydratase, the branched chain amino acid aminotransferase, the leuC subunit of 3-isopropylmalate dehydratase, or the leuD subunit of 3-isopropylmalate dehydratase with a compound to be tested; and (e) comparing the activity of the dihydroxyacid dehydratase, the branched chain amino acid aminotransferase, the leuC subunit of 3-isopropylmalate dehydratase, or the leuD subunit of 3-isopropylmalate dehydratase that has been treated with a test compound to the activity of an untreated dihydroxyacid dehydratase, the branched chain amino acid aminotransferase, the leuC subunit of 3-isopropylmalate dehydratase, or the leuD subunit of 3-isopropylmalate dehydratase, thereby selecting compounds with potential for inhibitory activity.